This invention relates to an apparatus for a crystal of a semiconductor, which is suitable for formation of a semiconductor crystal growth layer with precision as precise as a single molecular layer.
A metal-organic chemical vapor deposition process (referred to hereinafter as an MO-CVD process) and a molecular beam epitaxy (referred to hereinafter as an MBE process) are well known in the art as vapor phase epitaxial techniques for forming a crystalline thin film of a semiconductor. According to the MO-CVD process, elements of the III and V groups, which are sources, and hydrogen or like gas, which is a carrier, are simultaneously introduced into a reaction chamber to cause growth of a crystal by means of thermal decomposition. However, the thermal decomposition results in a poor quality of the crystal layer formed by growth. The MO-CVD process is also defective in that difficulty is encountered for controlling the thickness of the layer with precision as precise as a single molecular layer.
On the other hand, the MBE process is well known as a crystal growth process making use of a ultrahigh vacuum. This process, however, includes physical adsorption as its first step. Therefore, the quality of the crystal is inferior to that provided by the CVD process which makes use of a chemical reaction. For the growth of a III-V compound semiconductor such as GaAs according to the MBE process, such elements of the III and V groups are used as sources, and the sources themselves are disposed in a growth chamber. Therefore, it is difficult to control the amount of gases produced by heating the sources, to control the rate of vaporization of the sources and to replenish the sources, resulting in difficulty of maintaining a constant growth rate for a long period of time. Further, the evacuating device exhausting, for example, the vaporized matters becomes complex in construction. Furthermore, it is difficult to precisely control the stoichiometric composition of the compound semiconductor. Consequently, the MBE process is defective in that a crystal of high quality cannot be obtained.
It is a primary object of the present invention to provide an apparatus for forming a crystal of a semiconductor, which can cause growth of a high-quality crystal layer of high purity on a substrate with precision as precise as a single molecular layer.
In accordance with the present invention, there is provided an apparatus for forming a crystal of a semiconductor comprising a growth vessel enclosing a substrate, means for heating the substrate, means for evacuating the growth vessel to a ultrahigh vacuum, nozzle means connected to external gas sources for introducing gaseous molecules containing those of component elements of a crystal which should grow on the substrate disposed in the growth vessel, and radiation emitting means for emitting and directing radiation having a wavelength of 180 to 600 nm toward and onto the substrate, whereby a semiconductor crystal growth layer having a predetermined film thickness is formed on the substrate with precision as precise as a single molecular layer.
Thus, by the provision of the means for emitting and directing radiation having a wavelength of 180 to 600 nm toward and onto the substrate, the temperature of the substrate can be suppressed to a low level. Consequently, a semiconductor crystal of high quality satisfying the stoichiometric composition can be provided. Further, when a nozzle for introducing an impurity gas is additionally provided in the growth vessel, any one of desired layers can be doped with the impurity, and the resultant film has a very steep impurity concentration distribution, thereby providing a semiconductor device capable of operation at a very high speed.